1. Field of the Invention
The present invention relates to a battery charger for charging a secondary battery for energizing an electronic device such as a video camera, and more particularly to a battery charger suitable for charging a nickel-hydrogen-occluded-alloy battery (hereinafter referred to as an "Ni-MH battery").
2. Description of the Prior Art
Nickel-cadmium (Ni--Cd) batteries are widely used as secondary batteries for energizing relatively small electronic devices such as video cameras. While an Ni--Cd battery is being charged, the electric potential between the positive and negative electrodes of the battery is detected. When the detected potential is increasing, the battery is continuously charged because it is normally charged. When the detected potential starts dropping, the battery charging process is brought to an end as it has fully been charged.
Other secondary batteries which are more efficient than the Ni--Cd battery are under development in recent years. One of the new secondary batteries is an Ni-MH battery. The Ni-MH battery produces a large amount of heat while it is being charged. If the Ni-MH battery is charged until the potential between positive and negative electrodes thereof begins dropping, it is so heated that it could not be held by hand, and hence is very dangerous when manually handled. To avoid this drawback, it has been customary to use a temperature detector such as a thermistor on the Ni-MH battery for detecting the temperature of the Ni-MH battery. The Ni-MH battery has output terminals for producing detected temperature information indicative of the temperature detected by the temperature detector, as well as terminals connected to the positive and negative electrodes through which the battery can be charged and discharged. While the Ni-MH battery is being charged by a battery charger, the detected temperature information produced by the output terminals is monitored. When the detected temperature of the battery exceeds a predetermined temperature, it is necessary to stop charging the battery.
However, the Ni-MH battery of the above structure is complex and costly because of the temperature detector and its output terminals that are additionally included in the battery. The battery charger used to charge the Ni-MH battery is not compatible with Ni--Cd batteries now in use since the battery charger has additional terminals for reading the detected temperature information from the Ni-MH battery.